1. Field of the Invention
The present invention relates to an optical transmission system and particularly to a bit-rate-independent optical receiver that is capable of operating a bit rate in an independent manner.
2. Description of the Related Art
A light transmission system can adopt various protocols, such as FDDI (Fiber Distributed Data Interface), ESCON (Enterprise Systems Connectivity), Fiber Channel, Gigabit Ethernet, and ATM (Asynchronous Transfer Mode) for high-bandwidth and high-bit-rate communications. Fiber optics technology can adopt various bit rates of 125 Mb/s, 155 Mb/s, 200 Mb/s, 622 Mb/s, 1062 Mb/s, 1.25 Gb/s, and 2.5 Gb/s to supply the capacity to meet the demand for multimedia applications. In operation, the light transmission system adopts one set of protocols as a multiplexing format for using any number of bit rates. In this type of light transmission system, the bit rate of an optical signal is set previously to a specific rate, such that an optical receiver can be designed to match the incoming bit rate. The function of an optical receiver is to convert an input optical signal into an electric signal and thereafter restore the electric signal to the original data that is noise-free.
FIG. 1 is a block diagram illustrating a conventional bit-rate-independent optical receiver. As shown in FIG. 1, the prior art optical receiver includes an optoelectric converting unit 110, an amplifying unit 120, a bit rate sensing unit 130, a bit rate determining unit 140, a reference clock generating unit 150, and a CDR (Clock and Data Recovery) 160.
In operation, the converting unit 110 converts an input optical signal into an electrical signal in the form of a photosensitive signal 115. A photodiode may be used for the converting unit 110. The amplifying unit 120 amplifies the photosensitive signal 115 is outputted from the converting unit 110 to output an amplified photosensitive signal 125. The amplified photosensitive signal 125 has a standardized voltage level. The voltage level of the amplified photosensitive signal 125 is set to “1” at a maximum and “0” at a minimum.
The bit-rate sensing unit 130 generates an electrical signal with pulses proportionate, in number, to the bit rate of the standardized photosensitive signal 125. The electrical signal is filtered and converted into a digital signal to be outputted as a bit-rate sensing signal 138. The bit-rate sensing signal 138 comprises a voltage level proportionate to the bit rate of the photosensitive signal 125—i.e., the input optical signal. FIG. 2 is a block diagram illustrating the bit-rate sensing unit 130. As shown in FIG. 2, the bit-rate sensing unit 130 includes a buffer 131, a delay 132, a calculator 134, a filter 136, and an analog/digital (A/D) converter 137. The buffer 131 distributes the photosensitive signal 125 received from the amplifying unit 120 to the delay 132 and calculator 134. The delay 132 generates a delay signal 133 delayed from the photosensitive signal 125 by a desired time and applies the delay signal 133 to the calculator 134. The calculator 134 exclusively OR's the photosensitive signal 125, directly received from the buffer 131, with the delay signal 133 received from the delay 132, thus outputting a recognition signal 135. The recognition signal 135 includes the form of a number of pulses with a high level interval that is equal to the delay time of the delay 132. When the input photosensitive signal 125 varies in response to the change in the bit rate, the number of pulses in the recognition signal 135 varies. Hence, the variation in the number of pulses in the recognition signal 135 is proportionate to the bit rate of the photosensitive signal 125. The filter 136 low-pass filters the recognition signal 135 received from the calculator 134, then outputs a filtered signal. The A/D converter 137 converts the filtered signal, which is the form of an analog signal, into a digital signal to be outputted as a bit rate sensing signal 138.
With continued reference to FIG. 1, the bit rate determining unit 140 determines the bit rate of the photosensitive signal 125 in response to the voltage level of the bit rate sensing signal 138, then outputs a clock correction signal 145 that is indicative of the determined bit rate. Thereafter, the reference clock generating unit 150 generates a reference clock according to the clock correction signal 145. Based on the reference clock, the CDR 160 reproduces the data and clocks of the photosensitive signal 125 and outputs the reproduced data and clocks.
However, the above-mentioned conventional bit-rate-independent optical receiver has a configuration in which one bit rate sensing unit and one bit rate determining unit must be allocated for each optical signal. Consequently, a plurality of optical signals that are capable of handling different bit rates is required if the bit-rate-independent optical receiver is applied as the input section of an optoelectric cross-connect (OEXC) system. It is then difficult to individually manage the required multiple number of the bit-rate sensing units and the bit-rate determining units. Therefore, it is very difficult to implement a centralized configuration that can be implemented as the bit-rate-independent optical receiver using the conventional system architecture.
Furthermore, the conventional bit-rate-independent optical receiver has a problem in which it is difficult to modulize elements adapted to process signals and control those signal is processing elements. Accordingly, it is disadvantageous to facilitate the addition or removal of the signal processing elements upon demand.